Gene targeting is a general method to mutate or replace genes at specific chromosomal sites in intact cells. The method relies on the production of a site-specific double-strand DNA break at the locus of interest, which is subsequently repaired by recombination with a homologous transgene. Precision BioSciences has developed a method for the production of "custom" endonucleases capable of targeting such DNA breaks to unique chromosomal sites with genome-level specificity. One of these enzymes, EndoJAK2, was designed to recognize a site in the human JAK2 gene, which is associated with a wide range of myeloproliferative disorders. This proposal outlines a set of experiments to investigate the ability of the EndoJAK2 endonuclease to direct the modification of the chromosomal JAK2 gene in a human cell line. If successful, these experiments will not only promote the development of a novel cancer gene therapy but will validate a general method for in situ genome modification in human cells. [unreadable] [unreadable] [unreadable]